The present invention relates to the production of high yield pulps from wood or other woody lignocellulosic materials, such as chips, shavings and sawdust. More particularly, the invention is directed to a pulping process of the type wherein such lignocellulosic material is treated with pulping chemicals and the treated material is subjected to a mechanical defibration.
Various processes exist for production of chemimechanical and semichemical pulps from wood using pulping chemicals such as NaOH, Na.sub.2 SO.sub.3, Na.sub.2 S, Na.sub.2 CO.sub.3, and Na.sub.2 SO.sub.4. These processes produce pulp with properties which limit the use of these pulps for low quality and low price products such as corrugating medium, packaging grade, newsprint furnish, etc. Due to a limited fiber separation in pulping, high refining energy requirements are typical for these processes. Furthermore, processes such as chemimechanical pulping process (CMP) and neutral sulfite semichemical pulping process (NSSC) use sulphur-containing chemicals in pulping and thus encounter problems related to air and water pollution and corrosion due to the presence of organic sulfur compounds in the process vapors and water effluents.
In the pulping process disclosed in U.S. Pat. No. 4,116,758, for example, wood chips are first sulfonated to a high degree of sulfonation so as to produce a softening of the lignin in the wood sufficient to permit the wood chips to be readily difibrated into individual fibers by customary mechanical means. This high level of sulfonation which is about 85-90% of the maximum level of sulfonation that can be achieved on wood is obtained by cooking the wood chips in an aqueous solution containing a mixture sulfite and bisulfite in high concentrations. Since the attainment of the high levels of sulfonation required by such a pulping process involves the use of relatively high concentrations of cooking chemicals as well as of relatively heavy applications of cooking liquor on the wood, it becomes necessary for economic considerations to recycle the unreacted sulfite from the cooked chips.